Tin plate baking pan



Nov. 22, 1955 J. J. RUSSELL ETAI- 2,724,526

TIN PLATE BAKING PAN Filed April 18, 1950 2 Sheets-Sheet l NOV. 22, 1955J, J, RUSSELL ET AL 2,724,526

TIN PLATE BAKING PAN Filed April 18, 1950 2 Sheets-Sheet 2 United StatesPatent() TIN PLATE BAKING PAN John James Russell, Des Plaines, WilliamA. Beck, Itasca, and Jack Kollman, Chicago, Ill., assignors `to EkcoProducts Company, Chicago, Ill., a corporation `of Illinois ApplicationApril 18, 1950, Serial No. 156,672`

2 Claims. (Cl. 220-64) i This invention relates to a composite metallicstructure which may be fabricated into shaped metallic articles havingouter surfaces rendered highly absorptive to radiant energy whileretaining a relative high resistance to corrosion.

An object of our invention is the provision of a simple, direct, andthoroughly practical process for chemically treating tin and tin alloysby anodic treatment in various electrolytes, in order that the outwardsurfaces of the tin and tin alloys exhibit excellent heat absorption.

Another object of our invention is to `provide a fabricated articlewhich has been anodically treated wherein the surfaces of said articleshall be characterized by excellent heat absorption, high abrasionresistance, and continuity and uniformity of the`treated tin or tinalloy surfaces.` l

Another object of our invention is the provision that the heat absorbingcoating produced by the chemical process for treating the surfaces oftin or tin alloy shall be adherent and sutiiciently ductile in orderthat articles may be fabricated by mechanical, drawing, i forming,and/or bending operations.

Another object of our invention is `the provision that a shapedcomposite sheet metal fabricated article, such as a baking pan or aprocess tray, possess outerand inner surfaces having `relatively similarheat absorbing properties.`

Other objects and advantages of this invention will be made moreapparent as this description proceeds, particularly when considered inconnection with the accompanying drawings, in which:

Fig. l is a plan view of a baking pan embodying the invention;

Fig. 2 is a fragmentary cross-sectional view taken along the lines 2 2in Fig. l showing a schematic crosssection of the oxide coatings, alloylayers, tin layers, and steel base;

Fig. 3 is a photomicrographic cross-sectional View of a portion of a tinplate `area showing the oxide coating, tin layer, alloy layer, and steelbase (2500)( magnification);

Fig. 4 is a photomicrographic cross-sectional view showing the relativethickness of the oxide coating wherein the time of chemical treatmentapproaches about one minute (2500 magnification); and

Fig. 5 is a photomicrographic cross-sectional view showing the relativethickness of the `oxide coating which is substantially greater than thetin layer wherein the time of chemical treatment approached a fewminutes (2500)( magnification). j p

Referring now to the illustrated embodiments ofthis invention, themetallic shaped article, such as a baking pan, is designated generallybyreference numeral 2, its lower inside surface being shown at 4, and itsinner side surfaces by numeral 6. The outer side surfaces are designatedby numeral S and the outer bottom surface thereof by numeral 10.

The shaped article 2 as shown, is of the conventional folded end typebaking pan; although this type is being `used for illustration, theinvention shall not `be limited 2,724,526 Patented Nov. 2.2, 1955 ICC tosuch construction. The shaped article 2 is formed from a ilat compositesheet metal structure by the process of blanking into any desireddimensions and then forming the blank into a conventional folded endbaking pan in a manner well known in the art.

As conducive to a clearer understanding of certain features of theinvention, it may be noted that at this point tin articles havingsurfaces of tin or tin alloys shall also include tinplate, although thisinvention may be adapted to shaped articles formed of other basematerials, other than steel, such as copper, brass and the like.

Hot dipped tin plate is generally considered to be steel base metalcoated on its exterior surfaces with metallic tin wherein intermediateto the tin and steel interface, an iron-tin alloy composition is formed.Tin plate may be formed by either the conventional method of hot dippingor by electrolytically depositing the tin on the surface. The coatingweight of the tin is usually specified in pounds of tin per basis box,or in grams of tin per square metre of tin. The conversion of weight perbasis box to linear thickness depends on an assumed density which iscompensated by the fact that in. the hot dipped process there is agreater proportional thickness of the alloy layer formed than in theelectrodeposited process.

it is generally accepted that one pound per basis box is equivalent to0.0000606 inch thick of tin on each face of the tin plate. One andone-half pounds tin plate is generally assumed to be about 0.0000909inch thick. The proportional ratio between thickness and total weight ofa tin plate can be approximated from the relative proportion abovementioned for hot dipped tin plate.

In the case of the electrolytic process, tin plate thickness coatings of8 ounces to l0 ounces per basis box is generally accepted, although someapplications of tin plate may use less than 11/2 pounds but generallynot greater than 11/1. pounds per basis box.

ln the present invention, the anodic chemical treatment of tin plate isprimarily directed toward tin plate having the tin content greater thanabout 1% pounds per basis box which would primarily be adaptable to hotdipped tin plate, although the scope of the invention is also applicableto electrolytic tin plated articles.

The formation of oxide films of tin, as heretofore known may be producedby converting the tin into the oxide by subjecting the tin layer underoxidizing conditions, such as air, at elevated temperatures. Thisprocess of subjecting the tin to oxidizing conditions at elevatedtemperatures, has been conventionally used by the baking industry inconverting the tin layers to oxides of tin by placing formed articles oftin plate in baking ovens at an elevated temperature of approximately400 to 425 F. for a period in excess of about 4 to l2 hours. This hasbeen conventionally called the burning in or burning out process,wherein shaped articles 2, baking pans acquire a color ranging from theinterference films of light iridescent hues ranging in color fromyellow, blue through greens, and subsequently a surface coating having adegree of coloring may be obtained depending upon the chemical andphysical. characteristics of the tin plate surface in conjunction withvariable atmospheric conditions during the burning in cycle.

it has been found that these oxide films possess wide variations incolor characteristics; and it is highly de-` sirable to eliminate theseWide variations in order to produce uniform and consistent bread crustcolor. In addition, the tin-iron alloy intermediate to the tin layer andthe steel base is brittle and less corrosion resistant than the tinlayer. Normally, the tin-iron alloy is' substantially increased withrespect to the available metallic tin layer by these lengthy burning inoperations; and it is. highly desirous to reduce the "burning in time toabout 30 minutes to one hour. The baking industry finds it economicallyundesirable to subject the bread pan to long period of burning inbecause it ties up the baking ovens, baking pansas well as personnel ina non-productive operation and also produces variable results.

In addition, the temperature controls of the oven may vary considerably.It has been found that oven temperatures generally will exceed themelting point of the tin layer on the surface; as a result the tin-ironalloy layer will be substantially increased and the baking pans will besubsequently destroyed by the evaporation and decomposition of tinsurface layer. Also complete uniformity of crust color of bread is adefinite sales factor, and a baker who can produce a uniform loaf ofbread in the initial baking operation without the conventional burningin of a new set of pans, will increase his production capacity.

In addition, the adherence of oxide of tin, by the conventionalburning-in method, has been found to be very poor. Chemical factors aswell as atmospheric factors which control the adherence of oxides of tinare not well defined, and it is known that variations of the chemicaland physical composition of the surface layers of the tin will producevariation in adherence of the oxide coatings. Therefore, it is desirousto secure oxide coating formation having heat absorption which `isuniform and reproducible and at the same time to have a surface oxidecoating which will not tend to reflect heat energy that impinges uponthe surface of objects made of tin plate.

An outstanding object of the invention accordingly is the provision ofan economical industrially practical process for anodically treating tinplated articles wherein work of widely varying quality with respect tochemical and physical tin surface composition, as well as shapes ofmanufactured articles rnay be employed. In addition, the condition ofthe surface of the tin plate may be physically varied by mechanicaloperation; and under these conditions a uniform and constant color oxidefilm having excellent adherence and continuity, showing substantialreduction of spangling due to the breakdown of large crystal growth onthe tin surfaces may be produced within a pre-determined range of whollypractical conditions.

Referring now more particularly to the practice of this invention, thetin plate will normally have wide variations in the amount of tin on thesurface, and will vary widely with respect to surface conditions, suchas chemical composition, the presence of embedded organic foreignmatter, porosity of the tin layer, and the crystalline structure of thetin surface. It is undesirable to have a heat reflecting exteriorsurface in a bread baking operation, and the conventional burning inprocess will reproduce oxides of tin corresponding to the initialsurface condition.

The anodic chemical process of the present invention for treating tinsurface layers will substantially reduce the heretofore mentionedobjections; and in addition, will substantially remove objectionablecarbonaceous deposits within the porous tinplate, such as grease andoil. In addition, the anodic chemical process will tend to seal thepores of the tinplate, thus reducing the tendency toward porositycorrosion. In the chemical anodic treatment of tin plate, article 2, orproducts of various shapes and configurations, the tinplate layer iselectrolytically treated by using one or more of the plates, articles orproducts as the anodes in an electrolytic bath.

The electrolytic bath contains at least one or more or combinationsthereof, of a complexing reagent consisting of substantial amounts of(l) polybasic organic acids, such citric acid, picric, tartaric acid,oxalic acid, malic acid, maleic acid, and succinic acid; and (2)monobasic organic acids, such as acetic acid, lactic acid, propionicacid, benzene, sulphonic acid, trichloracetic acid, land salicyclicacid; and (3) non-oxidizing inorganic 4 acids, such as phosphoric acids,boric, molybdic, tungstic, and hydrofluoric acid; and (4) aqueoussoluble salts, such as the alkali metal and/or alkaline earth salts ofthe above mentioned organic acids or inorganic acids and other metallicsalt compositions. It has been found that combinations of the complexingreagents in aqueous solutions may be employed such as phosphoric acid,

combined with citric acid, phosphoric acid combinedl with sodiumphosphate; phosphoric acid combined with oxalic acid, acetic acidcombined with sodium citrate, and sodium phosphate combined with sodiumcitrate. The scope of this invention shall not be limited to thechemical composition of the complexing reagents.

In the anodic treatment of tin plated articles and products in theelectrolyte, it has been found advantageous to maintain currentdensities ranging from about 4 amperes per square foot to about 60amperes per square foot of tin surfaces undergoing treatment togetherwith a solution temperature of at least C.; and usually more, up to themaximum temperature which falls below the boiling pointof the solutionso as not to cause excessive evaporation. Under the chemical conditionsspecified together with time of. immersion, it is possible to obtain auniform meta stable tin oxide coating on tin plate having thicknessranging from about 3 to 5 micro inches to almost complete conversion ofthe free tin surface.

It is preferred to employ an electrolyte which by weight consists of atleast 0.5% up to about 30% of the complexing reagents wherein the pH ofthe solution lmay be adjusted if necessary, with the corresponding acidor combination of other acids so that the pH range may vary from about apH of 2 to a pH of about 8; and the remaining parts needed to form 100%by Weight being substantially of water. It has been found that theanionic stanno-complexes of tin are more readily formed when the pH isWithin the range of about two and one-half (2/2) to five (5).

One or more other tin plated articles 2, such as tin formed baking pansare made the anodes of the electrolytic solution and are subject toanodic treatment while maintaining a preferred solution bath temperatureof about 80 to 98 degrees C. and a current density of about 20 to 40amperes per square foot. The cathode may consist of shaped metalliccompositions, such as stainless steel or the lead lined tank may be usedas the cathode. Under such temperature conditions, the solution is foundto remain stable to the extent that the complexing reagent issubstantially retained in the bath with respect to weight' percentage.The complexing reagent is not substantially chemically removed from thesolution.

A uniform meta stable oxide of tin may be imparted to the tin surface ina short interval of time. The time of immersion together with solutionconditions are importantfactors with regard to the amount of meta stabletin oxidey formed by the anodic treatment. It has been found thatinterference films of about 2 to 6 micro inches thick may be formed byanodic treatment, in the electrolyte for an interval of timecorresponding to a few seconds. In addition, it has been found that in amatter of minutes the entire tin layer can be converted into a metastable oxide of tin.

As examples of other exceptionally stable, conductive and highlyeffective electrolyte solutions, and the related operating conditionswhich are employed for rapidly obtaining a uniform continuous metastable oxide of tin, the following treatments may be employed:

Treatment A Any remaining parts needed with the above to total 100%(percent) by weight, being substantially Water.

Bath temperature 90-100" C. Minimum current density 15 amperes per sq.foot.

Time of immersion 30 to 120 seconds.

Treatment B Percent Total t Weight of Bath Electrolyte PotassiumTartrate KQCiHrO Tartaric Acid HgCtHlO Any remaining parts needed withthe above to total 100 percent by weight, being substantially water.

Treatment C Percent Total Weight of Bath Electrolytc 5-20 PhosphoricAcid Any remaining parts needed with the above 100 percent by weight,being substantially water.

Bath temperature 80 to 100 C. Minimum current density 15 amperes per sq.foot. Time of immersion 30 to 120 seconds.

t total Treatment D Percent Total Weight of Bath Elcctrolyte TartaricAcid C OOI-I- (CHOH)2-C 00H 5-5l) Disodium Hydrogen Phosphate Anyremaining parts needed with the above 100 percent by Weight, beingsubstantially Water.

Bath temperature Sil-100 C. Minimum current density amperes per sq.foot. Time of immersion to 120 seconds.

to total Treatment E Percent Total W eight of Bath Electrolyte SodiumTartrate N a2C4H4O-2H2O 3-30 Disodium Hydrogen Phosphate NagHP Or.

Any remainingparts needed with the above to total 100 percent by weight,being substantially water.

Bath temperature 90-100 C. Minimum current density l5 amperes per sq.foot. Time of immersion 15 to 120 seconds.

Treatment F Any remaining parts needed with `the above to total 100`percent by weight, being .substantially water.

Bath temperature -100 C. Minimum current density 20 amperes per sq. ft.Time of immersion-; 30 to 120 seconds.

Treatment G Percent of Bath Sodium Oxalate NanCzOi 2-15 Phosphorlc AcidH3130; 3-10 Any remaining parts needed with the above to total 100percent by weight, being substantially water.

Bath temperature 70-100" C. Minimum current density 10 amperes per sq.ft. Time of `immersion 60 to 250 seconds.

Treatment H Any remaining parts needed with the above to total 100percent by'weight, being substantially Water.

Bath temperature l00 C. Minimum current density 20 amperes per sq. ft.Time of immersion 60 to 150 seconds.

An excellent meta stable oxide layer of tin was obtained on tin plate inthe instance of using Treatment A through TreatmentH, inclusive. Thescope of the invention should not be bound by any such quality ofchemical composition, nor by the specific proportions of acids, salts,and water given in the several illustrative examples of the treatment.

The `treated tin plate articles are then removed from the electrolytebath and rinsed with water in order to remove any occluded salts. Thetreated articles are then dried at slightly elevated temperature inorder to remove the adherent water. The treated tin plate articles uponwhich the meta stable `oxide of tin .is deposited thereon are thensubjected to an oxide conversion temperature condition whereby the metastable oxide of tin coating is then converted at elevated temperaturesto form the stable oxide of tin coating, primarily stannous oxide. Theelevated temperature range shall include approximately the melting point.of tin; although the preferred temperature range is from about 200 C.to about 230 C. This step in the process may be obtained by direct heatapplication to formulate the oxide conversion, although exposure to `airor to other oxidizing conditions will s0mewhat accelerate the oxideconversion at elevated tempep atures.

Theformation of the anionic stanno-complex of tin under electrolytictreatment will vary depending upon solution` compositions andconditions. Since tin exhibits amphoteric properties, it has been notedthat the meta stable oxide `of tinmay be formed. under alkalineconditions, although the electrolytic step in the process is preferredunder acid conditions (pH from 21/2 to 5), successful results have beenobtained under alkaline conditions. In addition it has been noted thatthe formation of the meta stable` oxide of tin may be formed rst byimmersion in an alkaline media and then subsequently into` an acid mediaor combinations thereof.

The complexing reagent forms the anionic stanno com plex which in turnis converted into the hydrate lof stannous oxide. The hydrate isunstable in fonn and is then partially converted into the meta stableform of stannous oxide. The meta stable stannous oxide coating exhibitsthe interference color on the surface of tin, and when the Vthickness ofthe meta stable stannous oxide appears to approach about seven (7) microinches, opacityof the lm begins to occur. By varying the time ofimmersion in the electrolytic bath, the blue-black meta stable stannousoxide is formed and at about 2 to 10 minutes immersion time under theaforementioned conditions it appears that the tin layer may be convertedtotally into the meta stable stannous oxide. Therefore, it is desirableto reduce the time of electrolytic immersion to about fifteen to ninetyseconds in orderk to retain a relatively high percentage of free tin.

The time interval for electrolytic immersion will vary depending uponsolution conditions. By reducing the current density and/ or thetemperature as well as the concentration of the complexing reagent, itis possible to increase substantially the time interval of immersion ofthe tin layer in the electrolytic bath. Therefore, the scope of theinvention shall not be bound by the specific time interval of immersion.

The meta stable stannous oxide exhibits various transitionalinterference colors approaching the blue-black. The blue-black is thenconsidered the end point of opacity; and it has been found that theinterference colors of red, purple, green, and blue-green will form,upon subjecting the then treated meta stable stannous oxide coating toconversion conditions at elevated temperature, the stable stannous oxidecoating, exhibiting a uniform olive green color. When using the termolive-green oxide of tin in the claims, it shall be understood that saidterminology shall define the color of the oxide per se.

Referring to Figure 2 in the drawing, the schematic fragmentarycross-sectional view represents a composite structure section of aprocessed shaped article 2, wherein the diagram illustratesschematically, the relative proportional thicknesses of the oxide tincoatings 22e, the tin layers 24e, the tin-iron alloy layers 26C, and thesteel base 28C.

It shall be noted that the oxides of tin coatings 22 Thisphotomicrograph illustratesV the thickness of thel oxide coating 22awherein a substantial portion of the tin layer 24a remains. Thecorrosion resistance of the tin plate has not been reduced substantiallywhile heat have approximately the same thickness on the inner surl faces6 as on the outer surfaces 8; thus, exhibiting similar heat absorptionproperties.

Figs. 3, 4, and 5, are single sectionphotomicrographs of a composite tinplate structure, such as a sheet of tin plate, or a section of thesurface of a shaped` article 2, wherein the photomicrograph correspondsto a magnification of about 2500. It shall be noted that thecrosssectons as shown in Figs. 3, 4, and 5, represent variablethicknesses of the oxide coatings 22 (a, b, and c), which may beobtained depending upon -the chemical composition, concentration of theelectrolytes as well as the time of immersion in the electrolytes'.These photomicrographs represent tin composition layers 24 (a, b, and c)including the tin alloy layers 26 (a, b, and c) respectivelycorresponding to one and one-half pound tin plate, having combinedthicknesses of approximately 0.000090 inch. The relative proportion ofthe amount of oxide coatings 22, with respect to the total amount of theinitial composite thickness of the combined tin layer 24 and the alloylayer `26, will depend on the initial coating Weight of the tin plate,and the chemical treatment to which the tin layer 24 has been subjected.

In Figs. 3, 4, and 5, the mounting 20 (a, b, and c) is the same. For thepurpose of securing a photomicrographic cross section, a copper mountingis generally used in order to back up the tin plate so that a welldefined boundary may be secured. In these instances, the mountings 20are used to back up the oxide coatings 22. l

Fig. 3 exhibits an opaque oxide coating 22avwhich has a thickness ofapproximately 15 micro inches thick. The oxide coatingi22a was formed onone and onehalf pound tin plate, using the aforementionedtreatabsorptive properties of the tin plate has been greatly enhanced.In addition the oxide coating 22 tends to reduce porosity corrosion ofthe tin layer 24, by substantially sealing the pores of the tin layerwith the oxide of tin coating 22, which exhibits chemical stability.

Fig. 5 is an illustrative photomicrographic cross. section of one andone-half pound tin plate, wherein the oxide coating 22e is substantiallygreater than the tin layer 24C. The oxide coating 22e was formed by theelectrolytic treatment of one and one-half pound tin plate usingTreatment C as previously described. The stable oxide coating 22e has athickness of approximately 60 micro inches as compared to the tin layer24e of approximately 25 micro inches.

In the accompanying Figures 3, 4, and 5, the relative thickness of thestable form of the oxide of tin coatings, 22 (a, b, and c), areproportional to thicknesses of the combined tin composition layers 24(a, b, and c), plus the alloy layers 26 (a, b, and c) respectively. Asthe thicknesses of the oxide coatings 22 (a, b, and c), increase therelative thicknesses of the combined tin layers 24 (a, b, and c), andalloy layer 26 (a, b, and c) decrease, respectively. This process isapplicable to other tin coating weights.

In examining the photomicrographic cross-section of the tinplate thathas been subjected to anodic treatment and temperature conversion steps,we have found that the stannous oxide coatings 22, may be varied inthickness ranging from a few micro inches to a thickness equivalent tothe entire layer 24, of the available free tin. Micro cross sectionexaminations of a one and one-half pound per basis box of untreated tinplate has a thickness of free tin layer 24 plus the tin-iron alloy layer26 of about 0.000090 inch. The tin-iron alloy layer 26 on hot dipped tinplate will be approximately equivalent to about .0.000010 to 0.000015inch thick. The amount of the tin layer 24 on each surface of one andone-half pound tin plate per basis box will approximate from 0.000060 to0.000080 inch thick. Allowing a sucient time interval to elapse in theelectrolytic immersion step, the tin layer 24 can be substantiallyconverted into the meta stable form of the stannous oxide. This appearsto be undesirable and chemical control ofthe amount of the meta stablestannous oxide coating 22 to be formed is imperative. It has been foundthat a tin oxide coating of approximately 0.000007 to 0.000030 inchthick will exhibit the preferred heat absorption on the surface of tinplate. Experimentally, it has been found that excellent bread baking wasproduced by converting up to about of the free tin layer 24, to the tinoxide coatings 22 without substantially decreasing the corrosionresistance of the tin plate. Thus, in one and one-half pound dipped tinplate, the conversion of 80% of the free tin `layer would leave at least12 micro-inchesv of free tin under the oxide.

`The thickness of the iron-tin alloy portion remains substantiallyunchanged from its thickness in the original tin plate. Although themeta stable oxide of tin coating may be used in the baking operation, wehave found that the stable oxide of tin coating 22 is the preferred tinoxide coating.

.The novel chemical treatment described in this process may be furtheraccomplished by using a fused salt, such as disodium hydrogen phosphatedodecahydrate or `magnesium phosphate, as the electrolyte, orcombinations thereof. One may consider this to be substantially anaqueous solution when the temperature of the electrolyte bath is within80 to 100 C. We have found that satisfactory results can be obtained byholding the bath temperature between 80 and 90 C. operating at a minimumcurrent density of about amperes for a square foot for a period of timeof about to 90 seconds. When the current density is held at aboutamperes per square foot, it has been found that the voltage applieddropped to about 2 volts. In this invention, the term complexing reagentshall also include a fused salt as described above.

`It shall be noted that the process as described may be applied to tinplate in sheet form wherein the anode may be a sheet of tin plate or ashaped article as defined by numeral 2. The tin plate or composite sheetstructure may be chemically treated by the treatments aforementioned;and then fabricated into shaped articles.

An alternative method of manufacturing these shaped articles 2, havingheat absorbing oxide coatings 22, formed thereon, may be accomplished byforming the tin plate and then subjecting the formed shaped 2, to theanodic chemical treatment by immersing the shaped article 2 in theelectrolyte. The cathode generally will have to be shaped into variousforms in order that a uniform oxide of tin coating 22 may be formed onthe tin layer 24; especially wherein the surfaces of the shaped article2 are deeply formed.

Another embodiment of this invention comprises the coating of the oxideof tin coatings 22 with a stable high temperature organic breadreleasing film. The organic bread releasing film may be applied prior tothe final temperature conversion step wherein the meta stable stannousoxide 22 may be coated with a stable high temperature organic breadreleasing film such as alkyl aryl silicones, polytetrauoroethylene,polytrifluoromonochloroethylene and other high polymer lm formingorganic substance and/or mixtures thereof. The film coated tin platedarticle 2, having either the inner, outer, or both surfaces coated, isthen subjected to the temperature conversion steps wherein independentlybut simultaneously the meta stable oxide coatings are converted into theopaque stable oxide of tin coatings 22, and the organic lm is cured.

Excellent adhesion of the organic bread releasing film to the treatedtin oxide coatings 22 has been obtained. It shall be noted that theorganic lm may be applied to either or both surfaces of the article 2,which has been previously converted into the stable oxide of tincoatings 22, and then be cured. The application of the organic breadreleasing iilm to the meta stable form of the oxide of tin coating andthen subjecting the combined meta stable oxide and the uncured organiciilrn to the temperature conversion step will eliminate one step in theprocess.

As many possible embodiments may be made of our invention and as manychanges may be made in the embodiments hereinbefore set forth, it is tobe understood that all matter described herein, is to be interpreted asillustrative and not as a limitation.

We claim:

1. A tin plate baking pan comprising; a shaped sheet structure includinga steel supporting base having an outer surface and tin compositionlayers bonded coextensively to said surface, said tin composition layerscomprising an inner portion consisting of an iron-tin alloy having athickness of approximately 10 to 15 micro-inches, an intermediateportion consisting of a metallic free tin layer bonded coextensively tosaid inner portion, an outer portion consisting of a uniform continuousheat absorbing tenaciously adherent opaque olive-green oxide of tinbonded coextensively to said intermediate portion, said outer portionhaving a thickness of about 7 micro-inches to 30 micro-inches.

2. A tin plate baking pan comprising a shaped sheet structure includinga steel supporting base having an outer surface and tin compositionlayers bonded coextensively to said surface, said tin composition layerscomprising an inner portion consisting of an iron-tin alloy having athickness of approximately 10 to 15 micro-inches, an intermediateportion consisting of a metallic free tin layer bonded coextensively tosaid inner portion, said metallic free tin layer having a thickness ofat least 12 micro-inches, an outer portion consisting of a uniformcontinuous heat absorbing tenaciously adherent opaque olive-green oxideof tin bonded coextensively to said intermediate portion, said outerportion having a thickness of about 7 micro-inches to 30 micro-inches.

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